1. Field of Invention
The present invention relates to a method of plasticizing a material for molding thermoplastic resin foam and a plasticizing apparatus. More particular, the present invention relates to a method of plasticizing a material for molding thermoplastic resin foam and a plasticizing apparatus arranged to perform the steps of: rotating a screw disposed in a screw cylinder such that movement in the rotational and axial directions of the screw is permitted to plasticize thermoplastic resin; causing inactive liquid, such as carbon dioxide solution or nitrogen solution in a super critical state to penetrate the molten resin or introducing inactive gas of carbon dioxide or nitrogen, the pressure of which is not lower than the super critical gas pressure into the screw cylinder which is in a hot state, the temperature of which is not lower than the super critical temperature, to create a super critical state in the screw cylinder so as to cause the inactive gas to penetrate the molten resin; and injecting the molten resin for molding a foam member to which the inactive fluid has penetrated into a cavity of a mold so that thermoplastic resin foam is obtained.
2. Related Art
A molding method or a molding apparatus has been disclosed in JP-A-8-258096, JP-A-10-230528 and so forth, in which thermoplastic resin is melted in a heating cylinder of an injection molding machine. Then, inactive liquid, such as carbon dioxide solution or nitrogen solution in a super critical state is caused to penetrate the molten resin. Then, the molten resin for molding a foam member to which the inactive fluid has caused to penetrate is injected into a mold. Thus, a thermoplastic resin foam member is molded. The apparatus for molding a foam member, that is, a manufacturing apparatus disclosed in JP-A-8-258096 incorporates a heating cylinder, a main screw disposed in the heating cylinder, a mixing screw disposed at the leading end of the main screw and an inactive-gas supply unit for supplying the inactive gas or inactive solution to the mixing screw portion. Therefore, when the main screw has been rotated to move the thermoplastic resin in the form of pellet to the leading end of the heating cylinder, the thermoplastic resin in the pellet state is melted owing to a shearing action caused from rotations of the main screw, heat produced owing to friction and heat applied from the outside of the heating cylinder. Then, the molten thermoplastic resin is furthermore uniformly melted by the mixing screw. When carbon dioxide solution in a super critical state is supplied in the foregoing state, the carbon dioxide solution penetrates the molten resin. The molten resin to which the carbon dioxide solution has penetrated is injected into the cavity of a mold by moving the main screw in the axial direction. When the molten resin has been cooled and solidified, the mold is opened. Thus, fine thermoplastic resin foam can be obtained.
The method of manufacturing thermoplastic resin foam disclosed in JP-A-10230528 incorporates two individual units one of which is a continuous plasticizing unit having a heating cylinder and a screw; the other one of which is an injecting unit having a plunger. Therefore, the two units enable the following thermoplastic resin foam to be obtained. That is, thermoplastic resin in the pellet state is supplied to the continuous plasticizing unit. Then, the screw is rotated. Hence it follows that the thermoplastic resin is melted owing to the shearing action caused from rotations of the screw, heat produced owing to the friction and heat applied from the outside of the heating cylinder. When carbon dioxide is supplied into the heating cylinder, a carbon dioxide penetrates the molten resin. The molten resin for molding a foam member to which carbon dioxide has penetrated is supplied to the injecting unit by moving the screw in the axial direction. Then, the plunger is operated to inject the molten resin into the mold so that thermoplastic resin foam is similarly obtained.
Either of the conventional manufacturing apparatuses enables the thermoplastic resin foam to be obtained. However, there are problems to be solved. Since the super critical temperature of carbon dioxide gas is 31xc2x0 C. and the super critical pressure is 7.4 MPa, there arises a problem of unsatisfactory sealing when carbon dioxide solution in a super critical state, the pressure of which is higher than the foregoing level, is injected into the molten resin in the heating cylinder. That is, since the thermoplastic resin to be supplied to the heating cylinder is in a solid state in the form of pellet, sealing cannot be realized by the thermoplastic resin in the pellet form. Thus, there arises a problem in that injected carbon dioxide solution leaks to the material supply opening. In particular, the foregoing manufacturing apparatus structured such that carbon dioxide solution is injected into the heating cylinder inevitably encounters the problem of unsatisfactory sealing. The reason for this lies in the high pressure of the molten resin in the heating cylinder which is 10 MPa to 30 MPa. Either of the foregoing manufacturing apparatuses cannot overcome the problem of unsatisfactory sealing. The screw of the apparatus for manufacturing fine thermoplastic resin foam disclosed in JP-A-8-258096 is composed of the main screw and the mixing screw disposed at the leading end of the main screw. on the other hand, the apparatus for manufacturing thermoplastic resin foam disclosed in JP-A-10-230528 is composed of the continuous plasticizing unit having the heating cylinder and the screw and the injecting unit having the plunger. Therefore, each of the apparatus must have too complicated structure, causing the cost of the manufacturing apparatus to relatively be enlarged.
To solve the foregoing problems experienced with the conventional technique, an object of the present invention is to provide a method of plasticizing a material for molding thermoplastic resin foam and a plasticizing apparatus. Specifically, an object of the present invention is to provide a method of plasticizing a material for molding thermoplastic resin foam and a plasticizing apparatus capable of injecting, into a screw cylinder, inactive solution, such as carbon dioxide solution or nitrogen solution in a super critical state or inactive fluid composed of inactive gas, such as carbon dioxide gas or nitrogen gas, the pressure of which is relatively low in spite of a level not lower than the super critical pressure and overcoming the problem of unsatisfactory sealing though inactive fluid, the pressure of which is not lower than the super critical pressure, is injected. Another object of the present invention is to provide a method of plasticizing a material for molding thermoplastic resin foam and capable of using a low-cost plasticizing apparatus having a simple structure.
To achieve the foregoing objects, a plasticizing method according to the present invention uses a screw cylinder having a material supply opening formed at the rear end and an injecting nozzle disposed at the leading end. Moreover, a screw is employed which incorporates a first metallized portion, a pressure-reducing portion and a second metallized portion. The inactive liquid in the super critical state or the inactive solution composed of inactive gas, the pressure of which is not lower than the super critical pressure, is injected to a position corresponding to the pressure-reducing portion of the screw. The injected inactive fluid is sealed by molten resin in the first and second metallized portions.
The foregoing first aspect enables the inactive fluid having a relatively low pressure level to be injected when the pressure is not lower than the super critical pressure. When the inactive fluid having the pressure not lower than the super critical pressure is injected, the sealing effect of the molten resin in the first and second metallized portions causes leakage of injected inactive fluid to the material supply opening or the portion more forwards than the screw cylinder to be prevented. Since the inactive fluid can be injected at a relatively low pressure, pressure resisting design of the super critical gas generating apparatus and piping and design for realizing sealing can easily be performed. The inactive gas injected into the screw cylinder is made to be inactive fluid in a super critical state in the screw cylinder, the temperature of which is not lower than the super critical temperature. The inactive fluid in the super critical state is allowed to penetrate and dissolved in the molten resin owing to the rotations of the screw. Thus, molten resin for molding a foam member can be obtained.
A second aspect of the invention has a structure that the inactive fluid composed of inactive fluid in a super critical state of inactive gas having the pressure not lower than the super critical pressure is injected into a starved feed portion which is formed in the pressure-reducing portion of the screw and which is not filled with the molten resin.
The second aspect causes the area of contact between the molten resin and the injected inactive fluid to be enlarged. Hence it follows that the effect to quicken and uniform penetration of the inactive fluid in the super critical state can furthermore be improved.
A third aspect has a structure that the starved state of the starved feed portion is controlled according to the amount of thermoplastic resin to be supplied to the supply portion of the screw.
The third aspect enables the area of contact between the molten resin and the super critical state or the inactive fluid brought to the super critical state to be controlled. Moreover, an effect can furthermore be obtained to control penetration of the inactive fluid in the super critical state.
A fourth aspect is structured such that injection timing of inactive fluid in the super critical state of inactive gas having the pressure not lower than the super critical pressure is controlled by using a timer.
The fourth aspect enables the injection timing of the inactive fluid or the inactive gas to be constant. As a result, the state of foaming can be uniformed and foam can be fined. Hence it follows that thermoplastic resin foam exhibiting high quality can be obtained.
A fifth aspect is structured such that the pressure in a second stage consisting of the pressure-reducing portion, a second compressing portion and a second metallized portion is made to be not lower than the super critical pressure. As an alternative to this, the pressure in a metering opening formed at the leading end of the screw cylinder is made to be not lower than the super critical pressure.
The fifth aspect enables foaming of the molten resin for molding a foam member in the screw cylinder to be prevented.
To achieve the foregoing objects, the plasticizing apparatus according to the present invention is an apparatus for plasticizing a material for molding a thermoplastic resin foam member comprising a screw cylinder and a screw disposed movably in the rotational direction and the axial direction. When the screw is rotated, the thermoplastic resin is plasticized. When the screw is moved in the axial direction, the molten resin which has been plasticized is injected into a mold. The screw cylinder has a material supply opening formed adjacent to the rear end thereof. An injection nozzle is disposed at the leading end. A gas supply opening for injecting inactive fluid which is inactive liquid in the super critical state or inactive gas, the pressure of which is not lower than the super critical pressure, is formed between the material supply opening and the injection nozzle. The screw incorporates a first stage having a front portion formed into a first metallized portion formed in a range from the rear end to the leading end to correspond to the screw cylinder; a second stage similarly having a front portion formed into a second metallized portion and a rear portion formed into a low pressure portion in which the capacity of screw grooves is enlarged; and a screw head portion formed at the leading end. The gas supply opening of the screw cylinder is formed at the position corresponding to the low pressure portion of the second stage of the screw.
The plasticizing apparatus according to the present invention incorporates the screw cylinder and the screw disposed in the screw cylinder such that rotation in the rotational direction and movement in the axial direction are permitted. Therefore, the structure can considerably be simplified and, therefore, the cost can be reduced. The gas supply opening is formed in the forward portion of the first metallized portion, that is, at a downstream position. Therefore, an effect can be obtained. That is, when inactive liquid in the super critical state or inactive gas having the pressure not lower than the super critical pressure is injected, any counter flow of the injected inactive fluid to the material supply opening can be prevented owing to the sealing effect of the molten resin in the first metallized portion. Moreover, the gas supply opening is opened in the low pressure portion of the screw. Therefore, injection is permitted when the pressure for injecting the inactive fluid is lowered. Therefore, furthermore complete sealing can be realized also thanks to the sealing effect of the molten resin.
A second aspect of the plasticizing apparatus has a structure that the screw incorporates a second stage formed into a multiple-flight structure. As an alternative to this, the screw has a mixing piece disposed at the front portion of the second stage thereof.
The second aspect enables penetration of the inactive fluid to quickly and uniformly be completed. Therefore, foam particles obtained by using the apparatus for plasticizing the material for molding the thermoplastic resin foam member can be fined. Moreover, aesthetically pleasing, fine and uniform foam particles can be obtained. As a result, thermoplastic resin foam excellent in strength can quickly be obtained.
A third aspect has a structure that the screw has a screw head portion provided with a ball-checking counterflow preventive unit.
The ball of the third aspect has a small inertia force and delicate response. Therefore, the molten resin for molding a foam member which has been plasticized and metered can be injected without any leakage. Hence it follows that thermoplastic resin foam excellent in an accurate weight and quality can be obtained.
A fourth aspect has a structure that the screw cylinder has a plurality of grooves formed in the axial direction and provided for the inner wall adjacent to the material supply opening.
The fourth aspect attains an effect to be obtained in that the frictional force between the inner surface of the screw cylinder and the thermoplastic resin can be enlarged and the plasticizing performance can be improved.
A fifth aspect has a structure that the gas supply opening of the screw cylinder is provided with a heat resisting and gas-permeable member.
The fifth aspect enables leakage of the molten resin to the outside to be prevented when supply of the inactive liquid or the inactive gas. Moreover, the gas permeable area can be enlarged as compared with one small opening. As a result, an effect can be obtained in that the inactive liquid or the inactive gas can quickly be injected.
Other and further objects, features and advantages of the invention will be evident from the following detailed description of the preferred embodiments taken in conjunction with the attached drawings.